1. Field of the Invention
The present invention relates generally to color cathode ray tube (CRT) display apparatus and more particularly to CRT displays used in applications under which the ambient light conditions vary over a very wide range. One such application is an aircraft cockpit wherein the ambient light can vary from direct, high altitude sunlight to almost total darkness. High contrast enhancement filter techniques which may be of the type disclosed in the present assignee's U.S. Pat. No. 3,946,267 are used to maintain the desired contrast ratios under such light ambients. More specifically, the present invention relates to CRT display apparatus; for example a shadow-mask type color CRT, for use in such ambient light conditions which automatically adjusts the focus of the CRT beams in accordance with a variable reference brightness and in accordance with the display writing technique being used, e.g., raster or stroke, in a manner to maintain a substantially constant display linewidth over the brightness range for the purpose of eliminating or substantially eliminating any roping and moire effects.
2. Discussion of the Prior Art
As is well known in the shadow-mask color CRT art, both "roping" and "moire" effects result from the interaction of the electron beam with the size, spacing and pattern of the individual primary color phosphor dots. A roping effect (a line which appears as a twisted fiber rope) results when the effective electron beam width does not span a sufficient area of the dot pattern. A narrow beam tends to isolate individual dots or dot patterns and at certain beam line orientations so that dots lie diagonally across the beam trace. A moire effect results when, given a fixed center-to-center line separation, the regular individual lines of the raster pattern are of a width such that the spacing between the lines interferes with the regular row and/or column pattern of the phosphor dots. This interference pattern will normally result when the effective raster line spacing produced by a particular line width approaches the row or column pitch of the dot matrix. Thus, in each case the only controllable factor, for any given CRT shadow-mask characteristic is the electron beam width.
In most prior art CRT display systems, such as for example, home and commercial TV's, where normal viewing ambient light conditions do not vary significantly or where if viewing is in high ambient light conditions mechanical shades or baffles are used to prevent direct sunlight from impinging upon the CRT face, essentially a fixed predetermined reference beam width or focus voltage is established based primarily upon the particular phosphor dot pattern and dimensions, to define a desired linewidth so that roping and moire is minimized and is thereafter left unchanged even with manual changes in brightness. Such techniques, while useful in home and commercial TV apparatus are not useful in CRT displays in which the brightness of the displayed symbology must be varied over a wide range to accommodate the wide range of ambient lighting.
In applications where a fixed display format is required other possible methods can be used to eliminate moire in a raster presentation. If a fixed linewidth is required the resolution of the phosphor dot pattern can be increased thus eliminating the moire effects. Furthermore, the orientation of the phosphor dots can be changed to the optimum configuration during the design of the CRT to minimize moire effects. Unfortunately the above solutions are technically difficult and very expensive.